Formamide mixtures as de-icing materials



Oct. 22, 1963 P. J. HEARST FORMAMIDE MIXTURES AS DE-ICING MATERIALS Filed May 11, 1959 FORMAMIDE AND SOLUTION D -l O (D SOLUTION 0 o o S.

FIG.

40 5O COMPOSITION- PER CENT (BY VOLUME) FORMAMIDE ID 0 INVENTOR:

PETER J. HEARST BY m ATTORW United rates harem 3,108,075 FQRMAMlDE MEXTURES AS; DEE-KENS MATERlALS Peter ll. Hearst, Uxnard, Calif, assignor to the United states of America as represented by the Secretary of the Navy Filed May 11, 1959, Ser. No. 812,564 3 Claims. (til. 252--7@) (Granted under Title 35, U5. Code (i952), sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for Governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to low freezing point compositions which are particularly applicable to the de-icing of aircraft runways and paved air fields.

Although snow can be removed from runways adequately by existing equipment, there is no very satisfactory mechanical manner of removing ice or counteracting its low traction. ice on runways is considered to be particularly troublesome when it occurs in patches. A uniform ice cover is not as hazardous, since allowance can be made by the pilot for the anticipated lack of traction. When a dry patch, which offers good traction, is interspersed with slippery patches of ice, a plane coming in for a landing and using its brakes can easily go into a spin and a crash can result. Ice patches are generally formed when there are alternate thawing and freezing conditions. The patches may cover relatively small areas and may be relatively thin.

There are several possible approaches to the problem of combating ice on runways. These may be classified as (a) physical methods and (b) chemical methods.

Physical methods include the following:

(a) The mechanical removal of ice which is difficult because of the strong adhesion of ice to most surfaces.

(b) The heating of runways to prevent formation of ice or to remove ice is expensive. Heat can be obtained in an indirect manner by use of dark surfaces. Such surfaces absorb more radiation than do lighter and more reflective surfaces and this radiation is transformed into heat. By covering ice with coal dust, its rate of melting has been quadrupled However, for practical purposes, heating methods in general are either too expensive or are too slow in operation The use of abrasives to increase friction is considered undesirable for aircraft runways since the loose abrasives are sucked into and damage the jet engines. When such abrasives are employed, they must be implanted into the ice by the use of heat or ice melting chemicals so that they will not blow off too readily.

Chemical approaches can be divided as follows:

(0:) Methods of melting ice.

(Z1) Methods of reducing ice adhesion and thereby facilitating its removal, and

(0) Methods of preventing ice formation.

Of these methods only the first appears to have been successfully appled to paved surfaces. The more common ice melting chemicals are calcium chloride and sodium chloride. These have been extensively employed to melt ice on roads but they are too corrosive to be employed on aircraft runways.

All three of the above chemical approaches have been investigated to some extent in connection with icing problems encountered on ships and aircraft. To melt ice by depressing its freezing point, de-icing fluids have been developed which can be sprayed on the aircraft. The Air Force de-icing fluid control formula consists of ethylene glycol, isopropyl alcohol, and dextrose. (Ether solutions containin various inhibitors have also been pro posed.

ilb'dfilli Patented Get. 22, 1953 line Methods of reducing ice adhesion to metallic or coated surfaces have been studied by a number of laboratories. The force of adhesion of ice to most surfaces is considerably greater than are the cohesive forces within the ice. When attempts are made to remove ice from a surface, the ice will ordinarily cleave and only part of it will be removed. The remainder which may be a relatively thin layer will continue to adhere to the surface.

Methods of preventing the formation of ice by chemical means again depend on the depression of the freezing point or on the lowering of the adhesion of ice. For example, a de-icing fluid which will cling to and remain on a surface will act as an anti-icing material; so that instead of freezing on such surface, any water or ice present will form a solution with the freezing depressants. Furthermore, ice will form less readily on a surface to which it has low adhesion than on a surface to which it has high adhesion.

Of the possible chemical approaches to uhe problem of combating ice on runways, the present invention is concerned with better methods and materials for melting ice. in connection with this problem, the requirements for a proper de-icing material are as follows:

(a) It must be capable of easy distribution manually or mechanically.

(b) It must be reasonably economical and the use of critical materials should be kept to a minimum.

(c) It should perform satisfactorily at as low a temperature as may be desired or which may be encountered.

(d) It must not be detrimental to jet-propelled aircraft.

('2) It must not be detrimental to pavement.

(f) it must be non-corrosive.

(g) It must be non-toxic.

(It) it must perform its function within a reasonable time.

Chemicals, which melt ice by freezing point lowering, do so by forming a solution of a lower freezing point than that of pure water. The lowest temperature at which an ice melting chemical can properly be effective is at the lowest freezing point which can be obtained for a solution of the particular chemical in water. Since the freezing point depression depends primarily on the number of molecules or ions of material in solution rather than on the total amount of material dissolved, an effective ice melting chemical should not only be very soluble, but should also have a low molecular weight. Inorganic salts, which consist of ions of low molecular weight and have a high solubility, are effective for certain uses, but as the salts are quite corrosive, they cannot be used on aircraft runways. This is due to the corrosive effects on the materials used in aircraft construction, such as aluminum, magnesium, and various other metal couples.

In view of the limitation to chemicals which are noncorrosive, the choice was therefore limited to organic compounds. Only the water soluble organic compounds of the lowest molecular weights would be expected to give sufficiently high freezing point depressions to make them potentially useful as ice melting compounds. Various low molecular weight alcohols would give the desired freezing point depressions, but were not further considered because of their volatility, flammability, and toxicity.

The ice patches which are to be removed are considered to be produced by alternate freezing and thawing. T herefore, it is likely that the temperature of the ice being melted will not be very far below its freezing point and that in most cases a chemical which is effective at -18 C. (0 F.) would be suitable. My invention contemplates a de-icing composition which will work effectively in the range of temperatures from -48 C. to 0 C., while fulfilling in all respects the requirements for a de-icing material as set forth above.

In order to accomplish the above, freezing point depressions were determined for the aqueous solutions of a number of compounds and compositions to establish the minimum temperatures at which the particular compounds could be effective in melting ice. Initial determinations were made with saturated aqueous solutions of solid compounds or with a 50% (by volume) aqueous solution of liquid compounds. The values obtained are listed in Table I, below:

Table I.-Freezing Points of Aqueous Solutions Crystulsolidifilization cation Material Con centrapoint, point, tion degrees degrees centicentigradc grade Sodium chloride Saturated -21 Calcium chloride. 51 Urea -11. Acetarnide -31. 5 Ammonium carbonate. 7. 5 Ammonium carbamate (fresh solu- -23 tion). Ammonium carharnatc-animonium carbonate. Ammonium earbamatc-ammonium -10. 5

carbonate-urea. Ammonium acetate -02 Do 57 Do -16 Methyl alcohol -43 Ethyl alcohol -29 Isopropyl alcohol -20 Acetone -17 Formamide, -31

-32 Mixture No. 5 Mixture N0. 0 -20 Mixture No. 7 1 -18 1 See Table 11 for composition.

The freezing points of various mixtures of formamide and water were determined to obtain a phase diagram as shown in FIGURE 1. Technical grade formamide was used which had a freezing point of 0 C. The freezing point lowering of formamide by various additive compounds was then investigated with the results shown in the following Table II.

Table Il.-Freezing Points of Formamide Solutions Crystallizasolidification point, tion point, Material degrees degrees centigrade centigrade Formarnide (practical) 0 90% formamide, 10% aeetamide 5. 5 -11 80% formamide, 20% acctarnidc 9. 5 -10 Formamide saturated with ammonium carbamate -6. 5 Formamide saturated with ammonium caron e -7 -24 90% tormamide, 10% ure -5 80% formamidc, 20% urea -12 70% formamide, 30% urea -7 -12 80% formamide, 20% ethyl ale0hol -10 -22 75% formamide, 25% ethylene glycoL -15 70% iormamidc, ethylene glycol -17 -53 Mixture No. 1 (60% formamide, 20% ace :1-

mide, 20% urea) -10 -23 Mixture No. 2 (00% lormarnide, 20% acetamide, 10% urea, 10% ethyl alcohol) -16 Mixture No. 3 (00% formarnidc, 10% acetamide, 10% urea, 10% ethyl alcohol, 10% ethylene glycol) -30 -44 Mixture No. 4 (70% formarnide, 10% acetamide, 10% urea. 10% ethylene glycol) -20 -40 Mixture No.5 (70%forman1ide, 15% urea, 15%

ethylene glycol) -18. 5 -48 Mixture No. 6 (70% formamide, 20% urea, 10%

ethylene glycol) -19 -28 Mixture N0. 7 (75% formamide, 20% urea, 5%

water) -22. 5 -35 A material which theoretically can melt ice at a very low temperature may be useless if it melts the ice too slowly. Comparative rates of melting ice were determined by observing the action of the potential dc-icing materials on an equal amount of ice at about -l0 C.

(14 F.). Seven milliliters of water were placed into a flat bottom vial of about 25 mm. inside diameter, and allowed to cool to -10 C. The ice disk produced was about 14 mm. high. The de-icing material was precooled to about 0 C. and enough material was employed to give a final mixture of the desired concentration (usually 50 percent) expressed in percent by volume for liquids and in percent by weight for solid materials. The de-icing material was added to the ice, and the vial was placed into a Cellosolve bath held at -10 C. The ice disks were inspected at 5-min. intervals through the walls of the bath, and the heights of the disks were measured. The liquid and ice interfaces were not smooth surfaces, and an average height was taken for the readings. To take readings, each of the six vials in the bath was in turn exposed to view by slowly rotating the bath. This was done carefully to minimize the agitation of the liquids.

The times required by various de-icing materials to half melt and to completely melt ice disks of equal weights or volumes are listed in Table III. For those materials for which several determinations were made, the average values, to the nearest 5 min., are listed. Deviations in the time intervals required by the same material in repeated runs were not more than 5 min. from the average value, when the time intervals involved were less than 50 minutes. DirTerences of not more than 5 min. in multiple runs, or not more than 10 min. in individual runs, may therefore not necessarily be significant.

Table IIl.Rates of Ice Melting Time required Acctamide 2 Over 32 hr.

Methyl aleohoL Ethyl alcohol- Formamide 15 rnin 40 min. Mixture N0. 4 15 min 45 min. Mixture No. 6. 55 min. Mixture No. 7. 55 min. Ethylene glyeoL min. Air Force deicing flu 35 min 75 min. Glycerol 60 min 3 hr.

The corrosive effects of aqueous solutions of formarnide on aluminum and copper were determined. Negligible corrosion on aluminum and slight effects on copper were observed. The ammonium salts on the other hand did react at various rates with copper to form blue solutions. In comparing the slippery nature of a smooth surface wetted with water, formamide, or ethylene glycol, it was observed that the latter material produced a much more slippery surface whereas no substantial difference was noted between the effect of water or formamide. Formamide on a runway would therefore give better traction than would ethylene glycol.

Flash points were determined by the Cleveland open cup method with the following results:

Table IV Flash point, Fire point, Material degrees degrees Fahrenheit Fahrenheit Ethylene glycol 255 255 Formamide 250 315 r Mixture No. 7 .1 330 350 areas-"75 The chemicals which were investigated for their freezing point lowering were those which were expected to give the best results. To help insure that no likely material would be overlooked, a comprehensive list of common organic compounds was scanned for chemicals with specific gravities higher than 1.02, high water solubilities, and relatively low molecular weights. From the freezing point data in Table i above, it appeared that ammonium carbamate, ammonium acetate, and acetamide were the more promising solid materials, and that methyl alcohol, ethyl alcohol, formamide and ethylene glycol were the more promising liquid materials.

Since considerable heat is required from the surroundings in order to melt ice, it may in certain cases be desirable to apply additional heat. if the application of heat is necessary to extend the useful temperature range of a de-icing material or to speed its action, it appears that the most effective method would be to heat the deicing material either before or during application. This method would be more practical for a liquid de-icing material which can be applied as a spray. If the flash point of the material is sufficiently high, there should be no danger of ignition.

The experiments to determine the rate of melting ice showed that methyl alcohol and ethyl alcohol do not perform nearly as well as might be expected from the freezing point data, and that formarnide is much superior to either of these alcohols. The differences in performance apparently are caused by the dilferences in specific gravity.

When a solvent of low specific gravity is placed on ice, 2. layer of solvent mixed with water will be formed at the interface. This layer will not be very effective in melting additional ice, and the water in this layer will mix with fresh solvent very slowly by diffusion. However, when the solvent has a high specific gravity, the solution formed at the interface will be lighter than the solvent and will mix with fresh solvent more rapidly by convection.

Among the solid chemicals avhose rates of melting ice were investigated, finely divided or powdered material was found to be less effective than coarser material. This lower effectiveness appeared to be due to the coalescing of the material as it was being wetted. Thus, previously wetted ammonium carbamate showed a higher rate of melting ice than did the dry powder. Increasing of the diameter of the ice disk from 1 in. to 3 in. did not materially affect the rate of ice melting of ammonium carbamate powder or of granular sodium chloride. Employing coarser material, 4 mesh to 8 mesh, markedly increased the rate of both of these chemicals. The rate of melting ice by ammonium acetate probably could also be further increased if coarser material were available.

From the above experiments on the rate of ice melting and from the freezing point determinations, it appeared that the most promising materials were formamide, ethylene glycol, and ammonium acetate. The first two of these would be easier to apply to runways since they are liquids and can be sprayed. The ammonium acetate, however, is effective at lower temperatures.

The main disadvantage of relatively pure formamide is its comparatively high freezing point. Fo'rmarnide which has been diluted by :water will melt at a lower tempenatu-re since the dissolved water will act as a freezing point depressant.

'I he dilution necessary to keep formamide from freezing at a particular temperature can be determined from the phase diagram for aqueous formamide, FIG. 1. Formamide in a concentration range of about 33 percent to 88 percent will not freeze at -18 C. (0 F.). If the concentration of formarnide is lower or higher and the temperature is brought down to -18 C., water or formamide, respectively, will crystallize out to form a slush. This crystallization will continue until the concentration of 33 percent or 88 percent, respectively, is reached. If the temperature lowering is continued, the crystallization will continue and the remaining solution will thus change in concentration. This change will follow the curve of the phase diagram, and it will continue as the temperature is lowered until the eutectic temperature of 48 C. (54 F.) is reached. At the eutectic point, no further change in concentration will take place, and the slushy mixture will freeze to a solid.

Ethylene glycol, while widely employed as an antifreeze for engines, has several disadvantages when thought of as a de-icing agent for runways. It has a lower rate of melting ice than does formamide and is more expensive. A surface wetted with ethylene glycol is more slippery than when formamide is used and thus the use of the former material is much less desirable.

Where it is necessary to melt ice at temperatures so low that formamide is not effective, ammonium acetate may be employed. A saturated solution of ammonium acetate freezes at -62 C. (80 F.), and ammonium acetate could be an effective ice-melting chemical at temperatures approaching this value.

Ammonium acetate, unlike formamide, is slightly corrosive to some metals. Because of its dissociation to form ammonia and acetic acid, ammonium acetate solution will accelerate the corrosion of copper by slowly dissolving the protective oxide coating. This attack is greatest in the vapor phase above the solution and this type of corrosion may possibly be quite small when there is good ventilation. At ordinary temperatures, the corrosion of aluminum in contact with ammonium acetate solution is higher than desirable, but at low temperatures this corrosion is negligible. If the ammonium acetate is employed at very low temperatures, there should be no corrosion problems.

Ammonium acetate has the advantage of having a cornparatively high rate of subliming. Material splashed on aircraft during take-off should therefore vaporize completely before a landing in a warmer climate where the corrosion rate may be higher. Furthermore, material left on airstrips should vaporize in a reasonable period of time.

Ammonium acetate has the disadavntage that it is a hygroscopic solid which in purified form is supplied as small crystals which cling together. The application of this material to a runway would be more difficult than the application of a liquid formamide mixture. The cost of ammonium acetate is considerably higher than that of formamide.

Lithium chloride has been investigated as a freezing point depressant for use in tire extinguishers and for use in aircraft de-icing fluids. Like calcium chloride, lithium chloride is quite corrosive, and furthermore, it is quite expensive. Ammonia would probably be objectionable for use on airstrips because it is a strong irritant of mucous membranes. Ammonia boils at 33 C. (28 F.) and it would be diflicult to apply at hi her temperatures. Ammonia has been investigated as a de-icing material for aircraft Windshields for use in flight.

Of all the formamide mixtures tested, Mixture No. 7, by weight of formamide, 20% by weight of urea and 5% by weight of water) seems to be the best and most practical, particularly from the standpoint of expense, lack of inflammable or slippery components, and crystallization point. Mixture No. 7 has the great ad vantage of having high flash and fire points as compared to the other materials listed in Table IV. Urea is comparatively inexpensive, hence Mixture No. 7 would be less expensive than pure formamide and, being a liquid, can be applied as a spray.

Having thus described my invent-ion, it should be understood that the foregoing illustrative examples are not to be considered as in limitation thereof. Many apparently different embodiments may be made without departing from the spirit thereof and/ or the scope of the appended claims, wherein I claim:

1. A fluid de-icing composition consisting essentially of 30% by weight of at least a 75% solution of formamide in water and 20% by weight of a solute taken from the group consisting of acetamide, ammonium carbamate, ammonium carbonate and urea, said composition having a high water solubility, a specific gravity greater than 1.0, and a low molecular Weight.

2. A fluid tie-icing material for melting patch ice on aircraft runways consisting essentially of a ternary solution of 60% by weight of formamide, 20% by weight of acetamide and 20% by Weight of urea, said acetamide and urea being dissolved in the formamide, said solution having a high water solubility, a specific gravity greater than 1.0, and a low molecular weight.

3. A fluid de-icing composition for melting horizontal 8 sheets of ice consisting essentially of 75% by weight of formamide, 20% by weight of urea, and 5% by Weight of water, said composition having a high water solubility, a specific gravity greater than 1.0, and a low molecular Weight.

References Cited in the file of this patent UNITED STATES PATENTS 1,687,094 Isermann Oct. 9, 1928 2,136,385 Kaufman et al Nov. 15, 1938 FOREIGN PATENTS 340,294 Great Britain Dec. 29, 1930 

1. A FLUID DE-ICING COMPOSITION CONSISTING ESSENTIALLY OF 80% BY WIEGHT OF AT LEAST A 75% SOLUTION OF FORMAMIDE IN WATER AND 20% BY WEIGHT OF A SOLUTE TAKEN FROM THE GROUP CONSISTING OF ACETAMIDE, AMMONIUM CARBAMATE, AMMONIUM CABONATE AND UREA, SAID COMPOSITION HAVING A HIGH WATER SOLUBILITY, A SPECIFIC GRAVITY GRETER THAN 1.0, AND A LOW MOLECULAR WEIGHT. 